NISTIR 7378

A Taxonomy of Definitions for 

Usability Studies in Biometrics

Ross J. Micheals

Brian Stanton

Mary Theofanos

Shahram Orandi

Information Access Division

Information Technology Lab

November 2006

 
NISTIR 7378

A Taxonomy of Definitions for 

Usability Studies in Biometrics

Ross J. Micheals 

Brian Stanton 

Mary Theofanos

U.S. Department of Commerce 

Technology Administration 

National Institute of Standards and Technology 

Information Technology Lab 

Gaithersburg, MD 20899 

Shahram Orandi 

MitreTek Systems 

Falls Church, VA 

November 2006

U.S. Department of Commerce 

Carlos M. Gutierrez, Secretary 

Technology Administration 

Robert Cresanti, Under Secretary of Commerce for Technology 

National Institute of Standards and Technology 

 William Jeffrey, Director 

 
1. Introduction 

Historically, usability research strives to focus on user behavior in a manner that is as system independent 

as possible. Although “ISO/IEC JTC 1/SC 37 Standing Document 2 — Harmonized Biometric 

Vocabulary, Draft N1779” provides a foundation towards common terminology, it currently lacks 

definitions specifically for use in usability studies that are also consistent with this practice. 

To address this gap, this document describes a taxonomy of definitions for use in usability studies of 

biometrics. The use of the term taxonomy was deliberately selected since the terms as presented here 

represent different classes of definitions. That is, practitioners are expected to further refine the 

terminology to best fit their particular needs. The taxonomy is designed to be general enough to 

incorporate a variety of world views, but specific enough to provide a common ontology for facilitating 

clear and concise communication. 

Whenever possible, the taxonomy was designed to fit within the aforementioned international 

Harmonized Biometric Vocabulary. However, it is emphasized that these definitions are distinct from 

existing biometric terminology in that each term is driven by user behavior as opposed to a systemoriented viewpoint. 

2. Taxonomy 

Definitions are presented here in traditional ISO convention; i.e., “the form of a definition shall be such 

that it can replace the term in context.” 

2.1 Presentation

The term presentation is defined as follows:

presentation 

the display of biometric characteristics to a sensor 

Mapping user behavior to the definition of a presentation is straightforward; consider the following:

EXAMPLE. The positioning of oneself for having a facial photograph taken is a presentation; i.e., 

the display of facial biometric characteristics to a visible light sensor. 

The definition of presentation refers to characteristics (plural) as opposed to a single characteristic 

because a user may present multiple characteristics simultaneously. 

EXAMPLE. The simultaneous placement of a left index, middle, ring and little finger upon a scanner 

platen (active sensing area) is the display of fingerprint characteristics to an optical fingerprint 

sensor; i.e., a left slap presentation. 

Simultaneous presentations are not necessarily constrained to any particular specific biometric modality. 

EXAMPLE. The positioning of oneself for having facial and iris photographs taken by a multimodal 

sensor may be considered to be a single presentation. 

 
Practitioners are expected to describe the relationships among different potential groupings of 

presentations for their application. 

EXAMPLE. The left slap may be viewed as a single presentation (the presentation of a hand), or as 

four presentations (the presentation of multiple fingers). 

Presentations may be further qualified as implicit or explicit: 

explicit presentation 

a presentation made with the presenter’s awareness

implicit presentation 

a presentation made without the presenter’s awareness

Most presentations are likely to be explicit. 

EXAMPLE. A person looking into a camera for a driver’s license photo is offering an explicit 

presentation. 

However, sensors may be situated beyond the awareness of a presenter. 

EXAMPLE. If a person was being recorded via a hidden camera, then the presenter would be 

offering an implicit presentation. 

Awareness of the sensor or system itself cannot be equated with awareness of the presentation. 

EXAMPLE. Some airport exit kiosks are capable of taking facial photographs with a fairly wide 

angle lens. A person may offer an implicit presentation if they are standing in an area in which 

they perceive that they are not being imaged, but are not far enough out of view to escape the 

camera’s field of view. 

Presentations may be further qualified as cooperative or uncooperative:

cooperative presentation

a presentation that the presenter provides willingly

uncooperative presentation 

a presentation that the presenter provides unwillingly

The definition of cooperative versus uncooperative depends only on the willingness to present. 

EXAMPLE. Suppose a person applying for a visa arrives at a consulate and presents their 

fingerprints to verify their identity. If they present their fingerprints willingly, then a cooperative 

presentation would occur. 

In some extreme cases, a cooperative presentation may be the display of biometric characteristics of 

someone else. 

 
EXAMPLE. A person entering the country under false documentation may be willing to present false 

biometric data to a fingerprint sensor (i.e., “spoof” the system). Even though their motivation may 

be to impersonate someone else, they may offer a cooperative presentation. 

Uncooperative presentations are more common in law enforcement scenarios. 

EXAMPLE. A suspect who flees a border patrol is caught and processed. If the suspect is reluctant to 

present their fingers for matching against a watch list, then an uncooperative presentation would 

occur. 

It is anticipated that most usability studies will be limited to cooperative presentations. Practitioners are 

therefore encouraged to assume that presentations are cooperative unless otherwise specified. 

2.2 Attempt

The term attempt is defined as follows: 

attempt 

the response to an opportunity to make a presentation 

NOTE. Presentations may span multiple attempts. 

A presentation is typically made to a functioning device. 

 

EXAMPLE. The presentation of a left slap to a ready sensor is an attempt. 

However, a system needs to be neither online nor operational for an attempt to occur. Simply by the 

virtue of being available, a sensor implicitly affords an opportunity to present. 

EXAMPLE. The presentation of a left slap to a broken sensor is an attempt. 

EXAMPLE. The premature presentation of a middle finger to a sensor not yet online is an attempt. 

For completeness, we presume that practitioners are likely to consider a lack of response to a nonoperational or offline system as not an attempt. 

EXAMPLE. Making no presentation of an index finger to an offline sensor may be considered as “no 

attempt.” 

The following examples illustrate how presentations may span multiple attempts. 

EXAMPLE. A fingerprint sensor is configured to automatically capture a left slap followed by a 

right slap. Suppose the presenter places their left hand on the scanner and that the system 

successfully captures an image and immediately continues by polling for a right slap. In this case, 

the presentation of the left slap spanned two separate presentation opportunities. It is assumed 

that each presentation opportunity corresponded to the sensor polling for a different biometric 

(left slap, then right slap). 

EXAMPLE. A camera takes images in succession. Each image may be considered as separate 

attempts, but the presenter may be considered as making only a single presentation. 

 
A user may respond to a presentation opportunity with a lack of presentation. 

EXAMPLE. An operator triggers a fingerprint scanner to capture, but the intended presenter 

becomes distracted and no presentation occurs within some specified time (i.e., a system timeout 

occurs). 

Like presentation, practitioners are encouraged to further constrain attempt according to their needs. 

EXAMPLE. The presentation of a left slap may be viewed as a single attempt to present four 

fingerprints, or, as four attempts each corresponding to a different fingerprint. 

An attempt may be further qualified as acceptable or unacceptable: 

acceptable attempt 

an attempt that fulfills the minimal capture requirements of a system 

NOTE. Acceptable attempts must yield the minimum required biometric data from a working 

system in a particular configuration. 

unacceptable attempt 

an attempt that does not fulfill the minimal capture requirements of a system 

Notice that capture requirements are associated with a system, as opposed to a sensor. This allows for 

more flexible definitions of what constitutes the minimal capture requirements (such as requiring 

approval from a human in-the-loop). 

EXAMPLE. An attempt occurs where the presenter has their photograph taken, but an operator 

rejects the image. Since the system has labeled the data as not meeting the minimal capture 

requirements, the attempt is unacceptable. 

EXAMPLE. An attempt occurs where the presenter has their photograph taken, but the lens cap was 

not removed from the camera. Since the system did not acquire an image that contains a face, the 

attempt is unacceptable. This example is only valid if the set of minimum capture requirements 

includes obtaining an image that contains a non-occluded, visible face. 

The biometric data required from a system depends on the system configuration. 

EXAMPLE. A sensor collects a face and iris image simultaneously. The system may be configured 

to first attempt to collect an iris image, and use the face image as a fallback upon failure to 

capture an iris image. Such a system has a different set of minimal capture requirements than that 

same system configured to require both a successful face and a successful iris image.

With the exception of a human in-the-loop, the biometric system, and not user behavior is what makes an 

attempt acceptable or unacceptable. 

 
2.3 Task

The term task is defined as follows: 

task 

a set of user behavior that defines an attempt 

Tasks typically correspond to the desired user behavior. 

EXAMPLE. The presentation of a left slap to a sensor may be a task. 

This definition of a task allows for the further qualification of attempts. 

conformant attempt 

an attempt that fulfills the requirements set out by a task 

non-conformant attempt 

an attempt that does not fulfill the requirements set out by a task 

A lack of action may be the cause of a non-conformant attempt, since a lack of action is a form of 

response. 

EXAMPLE. A user (presenter) does nothing when instructed to place their left index finger upon a 

fingerprint scanner platen. Through lack of action, a non-conformant attempt occurs.

User (presenter) behavior, as opposed to system behavior, distinguishes between a conformant and a nonconformant attempt. 

EXAMPLE. A user (presenter) that places their right index finger on a fingerprint scanner when 

instructed to place their left index finger makes a non-conformant attempt since they do not fulfill 

the requirements defined by the task of placing their left index finger. Regardless of whether or 

not the system detects that a fingerprint has been captured, the attempt is non-conformant.

Since the acceptability and conformance qualifiers are orthogonal, they generate four categories of 

attempts. 

1. acceptable conformant attempt 

EXAMPLE. The presenter behaves according to task and the system’s minimum capture 

requirements are satisfied. 

2. acceptable non-conformant attempt 

EXAMPLE. The presenter did not behave according to task, but the system’s minimum capture 

requirements were satisfied anyway.

 
3. unacceptable conformant attempt 

EXAMPLE. The presenter behaves according to task, but the system’s minimum capture 

requirements are not satisfied.

4. unacceptable non-conformant attempt 

EXAMPLE. The presenter did not behave according to task, and the system’s minimum capture 

requirements were not satisfied.

An acceptable conformant attempt implies the successful completion of a task because by definition, it is 

a response that simultaneously satisfies the task requirements while also producing the minimum required 

biometric data. 

The following scenario illustrates how the taxonomy may be used within the context of a simple usability 

experiment: 

EXAMPLE. Four participants (Participants A through D) are each instructed to perform a left slap

task. 

Assume, with respect to user behavior, that: 

• The set of user behavior that defines a left slap task is the placement of the left index, 

middle, ring, and little fingers onto the fingerprint sensor’s platen. 

• User behaviors occur at the appropriate times unless noted otherwise. 

Assume, with respect to system behavior, that: 

• The system is operational and configured in a manner for collecting left slap images. 

• The system may detect whether or not a hand is on the scanner and approves or rejects 

that data based on some built-in quality criteria.1 

• The system cannot detect which hand (i.e., left versus right) has been placed on the 

scanner. 

• If the sensor detects that hand has been placed on the scanner and approves the collected 

data, then the minimum capture requirements are met. 

Each participant illustrates a different category of attempt. 

Participant A presents a left slap to the fingerprint sensor, which the system detects and approves. 

The user behavior (the presentation of the left slap) yields an attempt that is acceptable (the 

system detects and approves the data) and conformant (the user fulfilled the left slap task). 

Participant B also presents a left slap to the fingerprint sensor, but the system rejects it. The user 

behavior yields an attempt that is unacceptable (it was rejected) yet conformant (the user 

presented the left slap as they were instructed). 

 

1 Although many sensors may have a high-degree of detection accuracy, it is possible for a system to fail to detect a 

presentation. The example does not illustrate this case. 

 
Participant C presents a right slap to the fingerprint sensor, which the system approves. The user 

behavior is acceptable (it was approved) but non-conformant, since the user behavior (the right 

slap) did not fulfill the task requirements. 

Participant D is distracted and makes no presentation to the fingerprint sensor and the system 

“times out”—i.e., no biometric data was collected within a specified interval. This attempt is 

unacceptable (no hand was detected on the scanner) and non-conformant (the response to the 

presentation opportunity was to do nothing). 

The following diagram is an illustration of the participant attempts. Each cell lists the category of 

attempt, the participant letter, and a summary of the action. 

acceptability 

acceptable conformant 

A 

left slap presented and approved by system 

unacceptable conformant 

B 

left slap presented but rejected by system 

conformance 

acceptable non-conformant 

C 

right slap presented and approved by system 

unacceptable non-conformant 

D 

no presentation, system timeout 

Acceptability changes across the vertical axis—the acceptable attempts are on the left (A and C), 

and the unacceptable attempts are on the right (B and D). Similarly, conformance changes across 

the horizontal axis—the conformant attempts are above (A and B) and the non-conformant 

attempts below (C and D).

3. Conclusion

This document presents a taxonomy of definitions for usability studies in biometrics. Unlike traditional 

biometric terminology, the terms defined here are primarily driven by user (as opposed to system) 

behavior. However, it is likely that the document could provide a foundation for a complimentary systemoriented taxonomy. Introducing a taxonomy, rather than a set of strict definitions, allows practitioners to 

further constrain the terminology presented here to better fit a particular application. The taxonomy’s 

ability to be adapted to a wide variety of edge cases provides evidence of its completeness and robustness.